Methods for equalizing WDM systems

ABSTRACT

A method of equalizing the channels of a WDM link comprises identifying an error threshold level BER Fail  for the BER defined for each signal S(j) in accordance with the channel rate, and determining the attenuation A(j) of, for example, the power P(j) of each signal S(j) transmitted over the WDM link. The transmitter powers are adjusted taking into account the attenuations determined for all channels. The attenuation A(j) for channel (j) is determined by first setting the power P(j) of all signals S(j) to a maximum P Max , attenuating the power P(j) of channel (j) until the BER reaches the threshold value BER Fail , measuring the power corresponding to the BER Fail  for that channel, and calculating the difference between the P Max  and P(j) Fail . The transmitter powers are then set according to the relationship P(j)=P Max −η(A(j)−A Min ), where η is 0.8 for a system with 3-4 channels. The method may be used for multi-channel systems with intermediate nodes where channels are added and dropped.

This is a divisional application of U.S. Ser. No. 08/997,822 filed Dec.24, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to multi-channel communication systems and inparticular to methods for equalizing WDM systems.

2. Background Art

High capacity optical transmission networks, such as those defined bythe SONET/SDH standards, can use wavelength-division multiplexing (WDM)to increase the information carrying capacity of the optical fiber. Inoptical WDM systems, a plurality of optical signals, each at a differentwavelength, are transmitted over a single optical fiber. The transmitterterminal consists of a like plurality of optical transmitters, typicallysemiconductor lasers, and an optical wavelength multiplexer, whichcombines all optical signals into a multi-channel signal before it islaunched over the optical fiber. Each transmitter operates at adifferent wavelengths and is modulated with a different data signal,either by directly modulating the laser or by external opticalmodulation.

At the receiver terminal, an optical wavelength demultiplexer separatesthe light received over the fiber according to the wavelength. Thesignal transmitted on each wavelength is then detected by a respectiveoptical receiver.

The WDM system reach, or the distance between the transmitter andreceiver sites, is limited by the attenuation or dispersion of thesignal along the optical fiber. The reach can be increased by placingoptical amplifiers at intermediate points between the terminals.Examples of optical amplifiers are semiconductor optical amplifiers, andrare earth doped fiber amplifiers. Optical amplifiers simultaneouslyamplify all optical signals passing through it, i.e. the multi-channelsignal, by amplifying the optical power by a gain.

Unfortunately, optical amplifiers exhibit a wavelength-dependent gainprofile, noise profile, and saturation characteristics. Hence, eachoptical signal experiences a different gain along the transmission path.The amplifiers also add noise to the signal, typically in the form ofamplified spontaneous emission (ASE), so that the opticalsignal-to-noise ratio (OSNR) decreases at each amplifier site. The OSNRis defined as the ratio of the signal power to the noise power in areference optical bandwidth.

Furthermore, the optical signals in the co-propagating channels havedifferent initial waveform distortions and undergo different additionaldistortions during propagation along the fiber. As a result, the signalshave different power levels, OSNRs, and degrees of distortion when theyarrive at the respective receivers, if they had equal power levels atthe corresponding transmitters.

WDM networks, and particularly SONET/SDH WDM networks, are widely spreadand the custom demand for these networks is growing fast. They providefaster bit rates, and are more flexible in terms of the bandwidth perchannel and complexity than the previous single-channel systems. Networkproviders are looking for features such as user-friendly installation,operation and maintenance, and thus, an equalization procedure that issimple and reliable will greatly simplify the set-up and hence reducethe maintenance costs of the communication system.

It has been shown that the OSNRs at the output of an amplified WDMsystem can be equalized by adjusting the input optical power for allchannels. For example, U.S. Pat. No. 5,225,922 (Chraplyvy et al.),issued on Jul. 6, 1993 to AT&T Bell Laboratories, provides for measuringthe output OSNRs directly and then iteratively adjusting the inputpowers to achieve equal OSNRs.

FIG. 1 shows a block diagram of a four-channel unidirectional wavelengthdivision multiplexed (WDM) transmission link deployed between terminals11 and 17, using OSNR equalization according to the above identifiedpatent.

There are four unidirectional channels λ(1)-λ(4) illustrated on FIG. 1,carrying traffic in the direction West-to-East. A short discussion ofthis method follows for a better understanding of the present invention.Terminal 11 comprises transmitters T₁ to T₄ and terminal 17 comprisesreceivers R₁ to R₄, connected over optical amplifiers 10, 20, 30, 40 and50 and fiber spans 10′, 20′, 30′ and 40′. The optical amplifiers arearranged at a suitable distance from each other, typically 100 km, tocompensate for the attenuation of the signal with the distance. Anoptical amplifier amplifies all four signals, as it is well known.

The lasers of the transmitters T₁ to T₄ are modulated with signals D₁ toD₄, respectively, to produce optical signals S₁ to S₄. A multiplexer 13at the site of terminal 11, combines optical signals S₁-S₄ into amulti-channel signal S, which is amplified in post-amplifier 10 beforebeing launched over the transmission link. At reception, themulti-channel signal is amplified by pre-amplifier 50 and separatedthereafter into signals S′₁-S′₄ with demultiplexer 15. Each receiver atterminal 17 converts the respective optical signal into an outputelectrical signal D′₁-D′₄, corresponding to input signals D₁ to D₄.

The U.S. Pat. No. 5,225,922 teaches establishing a telemetry linkbetween two terminals 11 and 17 of a transmission network, for providingthe measurements obtained at one terminal to the other. The patentindicates that the telemetry link may be provided with a control unit 5(a microprocessor) that receives the measured input powers of signals S₁to S₄ and the total output power or OSNR of multi-channel signal S, andadjusts the input power accordingly. This method also takes into accountthe known relative values of the gain for each channel. However, themethod disclosed in the above patent has three disadvantages: (1) itequalizes OSNR, which is only one parameter of several that affect theperformance of an optical transmission system, (2) measuring the OSNRrequires additional equipment, such as an optical spectrum analyzer,outside of the SONET/SDH standards, and (3) it cannot be used toequalize systems where channels with different wavelengths carry trafficwith different bit rates, since in such cases each channel has differentOSNR requirements.

FIG. 1B shows the optical spectrum of a 4-channel WDM system, showinghow the power of the channels varies with the wavelength.

As indicated above, in a typical WDM system the co-propagating channelsdo not have the same performance in terms of bit error rate (BER),because of different component losses, different transmitter andreceiver characteristics, different path distortions, and also becausethe gain and noise of optical amplifiers in the system arechannel-dependent. The BER is the ratio between the number of erroneousbits counted at a site of interest over the total number of bitsreceived.

There is a need for providing a method for equalizing WDM systems thatis more accurate and easier to implement than the current methods.

SUMMARY OF THE INVENTION

It is a primary object of the invention to provide a method forequalizing the BER performance of a WDM system, that alleviates totallyor in part the drawbacks of the prior art methods.

It is another object of this invention to provide a method forequalizing a multi-channel communication system based on monitoring theBER values for all co-propagating channels.

It is still another object of the invention to provide a method forequalizing a WDM system that corrects the performance differences of thenetwork elements without special instrumentation nor physical access toremote terminal sites.

Still another object of the invention is to determine the margins to thefailure point of all channels, regardless of their bit rates, which isan important parameter for the customer when deploying the network.

Accordingly, there is provided a method for equalizing the performanceof (J) transmission channels of a WDM link connecting a first terminaland a second terminal, comprising the steps of, (a) identifying an errorthreshold level E(j)_(Fail) of an error count indicator E(j) for asignal S(j), the E(j) characterizing the distortion of the signal S(j)between the first and the second terminal, (b) determining anattenuation A(j) of a parameter of interest P(j) of the signal S(j)between the first and the second terminal, (c) repeating steps (a) to(b) for all the J channels of the WDM system, (d) at the first terminal,adjusting the parameter P(j) according to all the attenuations A(j), and(e) repeating step (d) for all the signals S(j) for obtainingsubstantially equal values of the parameter for all the J signals at thesecond terminal.

Further, there is provided a method for equalizing a plurality of (J)signals S(j), travelling on a WDM link between a first terminal and asecond terminal comprising the steps of, for each channel λ(j) of theWDM link, measuring a distance to failure A(j) for a parameter P(j) of asignal S(j) travelling on the channel λ(j), and adjusting the parameterP(j) at the first terminal for obtaining equal distances to failure forall J channels.

Advantageously, equalizing the BER value for all channels is preferableto equalization of any other parameter such as OSNR, in that the BERvalue accounts for all factors that affect the signal in both itselectrical and optical states. The BER is the ratio between the numberof erroneous bits counted at a site of interest over the total number ofbits received, giving a measure of all errors introduced into a signalalong an entire transmitter-receiver link.

Furthermore, some systems (for example SONET/SDH) are specified in termsof the BER and therefore the BER value is available at reception.

The method according to the invention performs field equalization tooptimize a system in the field, and therefore a higher system margin isused than for equalization based on the average system parameters.

In addition, the method of the present invention does not requirenecessarily simultaneous access to both the transmitter and receiverends, but requires physical access to both, one or none of the terminalsites. The method could be automated by a software interfacing betweenthe terminals. Requiring simultaneous access to both terminal sites isdisadvantageous, because it requires at least two persons communicatingover long distances.

As indicated above, OSNR alone does not accurately characterize thesystem performance. The degradation of a signal is, on the other hand,fully expressed by the BER (bit error rate), which by definitionaccounts for all above listed signal degradation factors.

Furthermore, SONET/SDH systems are typically guaranteed in terms of aminimum BER requirement at the system end of life (EOL) and as such theBER measurement is generally available at any receiver site. Toguarantee the EOL performance, there must be enough margin built in thesystem at the start of life (SOL), since many of the system parameterschange in time or with environmental changes.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments, as illustrated in the appendeddrawings, where:

FIG. 1A is a block diagram showing a four-channel amplified WDM system;

FIG. 1B shows an example of the optical spectrum of the WDM system;

FIG. 2A is a block diagram of an optical receiver provided withperformance monitoring;

FIG. 2B is a block diagram of an optical transmitter;

FIG. 3 is a block diagram of an amplified two-channel system forequalization of the received power according to the invention;

FIG. 4 is a flow-chart showing the method of the invention forequalizing the system of FIG. 3;

FIG. 5 is a flow-chart showing another method for equalizing amulti-channel WDM system;

FIG. 6 is a block diagram of an amplified two-channel system fordetermining the various margins according to the invention;

FIG. 7 shows a WDM system with multiple terminal sites, whereequalization of the channels is performed automatically;

FIGS. 8A, 8B and 8C are flow-charts showing methods for equalizing amulti-channel WDM system with multiple terminal sites.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a method for equalization of atransmission link in terms of BER, which is a more accurate and easiersolution than what prior art provides. Also, this invention provides formeasurement of an optimum margin to failure for all the channels at thesystem SOL.

The invention is applicable to networks equipped with performancemonitoring capabilities. More precisely, the method of the invention isapplicable to modern WDM transmission systems, which are in generalprovided with means for measuring the system BER at various networkelements of interest.

The invention is described next using examples of SONET/SDH WDMtransmission systems, but it is to be understood that it may also beapplied to other technologies. The BER of a SONET/SDH system can bemeasured using the standard feature of this systems called theperformance monitor (PM). In the following, a block diagram of a typicalreceiver and a typical transmitter for the transmission systems to whichthis invention pertains are provided for a better understanding of theinvention.

A block diagram of a typical receiver is shown in FIG. 2A. An opticalreceiver generally comprises an optical-to-electrical converter 2, thatcould be an avalanche photodiode (APD), or a high performance PINphotodiode. A data regenerator and clock recovery block 3 extracts theinformation from the converted signal, based on a threshold levelV_(Th). The threshold is selected such as to provide the best error ratefor a predetermined signal power level. For example, the levels overV_(Th) are interpreted by the receiver as logic “1”s, while those under,as logic “0”s. The errors in regenerated data D₁′, namely the BER value,are counted using an error detector 4.

It is known to generate a control code at the transmission site which isthen transmitted with the information along the communication link.Error detection is based in general on comparison between thetransmitted and the received control code.

For example, the error detection in SONET/SDH determines the BER of therespective signal based on the information in the B1 and B2 fields ofthe transport overhead of the SONET/SDH frame, as well as field B3 ofthe path overhead for the respective section, line and path.

The threshold level V_(Th) applied to data regenerator 3 may be adjustedwith a controller 6, so as to obtain BER values under a provisionedvalue of the respective system. The error count and control data areinput to a performance monitor 7, connected to some or all remoteelements of the network over a bus 60.

As the requirement for essentially error free operation for fibersystems became more stringent, sophisticated performance monitors areprovided at the receiver site, which perform optimization routines forlowering the BER of the recovered signal.

Current transmitters are also able to monitor performance, as shown inthe block diagram of FIG. 2B. A transmitter, e.g. T₁ comprises a CW(continuous wave) laser 12, which is fed into the external modulator 8and is modulated with e.g. a signal D₁, to provide the optical signalS₁. Before transmission, this signal is amplified in an EDFA (erbiumdoped fiber amplifier) 32, which is provided with a pump laser 42 of acontrolled power and wavelength. The input, output and backplane powerof laser 42 are measured by diverting a fraction (approximately 3%) ofthe optical signal with taps 31, 33, and 34, respectively, andconverting the respective fraction into a corresponding electricalsignal with O/E converters 35, 36 and 37. The measurements are processedby a transmission controller 9, which in turn adjusts the pump laser 42and the external modulator 8, and communicates with the performancemonitor 7 and other remote elements of the network over bus 60.

In this way, the SONET/SDH performance monitor allows the user toremotely measure the channel performance anywhere in the network.

The equalization of the channels according to the invention is based onequalizing the BER values measured in a point of interest of thetransmission link, so that it accounts for all factors affecting thesystem performance. Since bit errors occur at random times, a minimumnumber of errors, e.g. 10, is required to estimate the BER for arespective transmission channel within an acceptable confidence level.For example, if the testing interval is chosen to be 1 minute and thebit rate is 2.5 Gb/s, then 15 errors/minute corresponds to a BER of10⁻¹⁰. In general, the network owner prepares BER-Power curves for allchannels at installation, so that these curves are also available forequalization purposes.

To accelerate the procedure, equalization is preferably done at a highBER. Since it is difficult to adjust the system parameters to obtain anexact value for the BER, the failure rate is defined within an order ofmagnitude for the BER. For example, the point of failure for a channelis defined herein when between 2 and 150 errors are counted in a minute,which corresponds to a BER range from 1.33×10⁻¹¹ to 10⁻⁹ for a 2.5 Gb/ssystem. It is to be noted that the definition of failure point for theequalization procedure is different from the guaranteed minimum BERrequirement of the system in operation.

The method according to the invention is next described for thetwo-channel system of FIG. 3 with regard to the flowchart of FIG. 4.

FIG. 3 illustrates a two-channel unidirectional amplified transmissionlink connecting transmitters T₁ and T₂ at terminal 11, with receivers R₁and R₂ at terminal 17; however, the invention equally applies tobi-directional systems with more than two channels.

The link includes fiber spans 10′, 20′, 30′ and 40′ connecting opticalamplifiers 10, 20, 30, 40, and 50 for amplifying channels λ(1) and λ(2).We note in the following the optical power output by transmitter T₁ onchannel λ(1) with P(1)_(T), the loss introduced by span 20′ to signal S₁travelling on channel λ(1) with L(1)_(S), the power of signal S₁ at theoutput of amplifier 20 with P(1)_(A), and the power at the input ofreceiver R₁ with P(1)_(R). BER(1) is the bit error rate measured afterdetection of the signal at the output of receiver R₁, while BER(2) isthe bit error rate measured after detection of the signal at the outputof receiver R₂. Individual notations are used for BER(1) and BER(2) fordistinguishing the channels in the process of describing theequalization method of the invention.

Same definitions apply to elements and parameters for channel λ(2), orto any other channels that a transmission system under consideration mayhave.

In this specification, the received power margin of a channel is theamount that the received power can be reduced until the channel fails.The transmitter power margin of a channel is the amount that thetransmitter power of a channel can be reduced until the channel fails.Similarly, the amplifier power margin of a channel is the amount thatthe output power P(j)_(A) of one or more of the amplifiers can bereduced until the channel fails, and the span margin of a channel is theamount of loss L(j)_(S) that can be added to one or more of the fiberspans until the channel fails, where (j) is the range of the channelbetween 1 and J, the total number of channels of the WDM system.

In the embodiment of FIG. 3, the equalization is performed manually atthe transmitter site 11, based on a the BER curves prepared atinstallation, that predict how BER varies with the power input to thelink. Alternatively, the actual readings of the BER(1) and BER(2) may beused, if the network is provided with a communication channel betweenthe transmitter site 11 and the receiver site 17.

For example, the communication between the sites may take place along abidirectional service channel illustrated in FIG. 3 by dotted line 22. Anetwork monitoring unit may be connected over this channel forprocessing the information received from terminals 11 and 17, or for anyother network elements of interest in the respective link.

FIG. 4 shows a flow-chart of the method of the invention for equalizingthe system of FIG. 3. First, the link is installed/commissioned to workin the current operating point, without equalization, as shown in step100. We note the range of a channel with (j) and the total number ofchannels with (J). In the example of FIG. 3 J=2 and (j) takes the values1 and 2.

In step 105, a BER failing point is defined for all channels,corresponding to a BER that is denoted with BER_(Fail). As indicatedabove, for example, the failing point of a channel is defined by a rangeof BER values or a fixed value of at about BER_(Fail)=10⁻⁹. The chosenfailure point depends on the bit rate and system requirements.

The transmitter-receiver pairs are identified, from the transmittersite, namely it is identified which transmitter and receiver communicatealong a channel; T₁ and R₁ operate on the frequency of channel λ(1), andT₂ and R₂ operate on the frequency of channel λ(2). This is illustratedin step 110.

Then, the output power of both transmitters is set at maximum, P_(Max);which is the maximum power of the weakest channel. The maximum valueP_(Max) or P_(Max) is measured and recorded, as shown in step 115.

With one channel at the maximum power, e.g. with transmitter T₂transmitting at P_(Max), the power of the first transmitter T₁ islowered until the first channel fails, as shown in step 125 for j=1.This measurement takes place under operating conditions of the link, andas such it accounts for the rate of transmission and other parameters ofthe respective channel. The value of the power for which the channelfailed is denoted with P(1)_(Fail).

An added attenuation A(1) is determined in terms of the power level instep 130. A(1) is calculated as:

A(1)=P _(Max) −P(1)_(Fail)  (1)

Then, in step 135, the power of T₁ is increased back to P_(Max).

This measurement is repeated for T₂ in steps 125-145. Again, P(T₂) isreduced until BER(2)_(Fail) is obtained at the receiver R₂. An addedattenuation A(2) is determined from the measured P(2)_(Fail) and P_(Max)according to the relation:

A(2)=P _(Max) −P(2)_(Fail)  (2)

and the power of T₂ is increased back to P_(Max) in step 135. If thesystem has more than two channels, i.e. j>2, steps 125-145 are repeatedfor each channel (j) and the respective attenuation is determined andrecorded.

Attenuations A(1) and A(2) are used to determine the optimum transmitterbias level of all channels, i.e. to select the operation point of thetransmitter, shown in steps 150 to 170 for the example of FIG. 3. A(1),in dB, is the added attenuation required to fail channel λ(1), and λ(2),in dB, is the added attenuation to fail channel λ(2). The strongerchannel can now be identified, as shown in steps 150 and 160 todetermine the required bias for the transmitters. The power of thetransmitter of the weak channel is then set to the maximum, and thepower in the stronger channel is adjusted so that the difference betweenthe output powers of the two transmitters equals the required powerdifference.

More precisely, the system should be biased as follows:

If A(1)>A(2), then the channel λ(1) should be attenuated by:

(A(1)−A(2))/2 (in dB)  (3)

and the channel λ(2) set at full power, as shown in steps 150 and 155.

If A(1)<A(2), then the channel λ(1) should be set at full power and thechannel λ(2) should be attenuated by:

(A(2)−A(1))/2 (in dB),  (4)

as shown at 160 and 165. For example, if channel λ(1) requires 10 dBattenuation to fail, and channel λ(2) requires 6 dB attenuation to fail,then 2 dB attenuation on channel λ(1) should be used during operation.

Step 170 accounts for the case when the two attenuations are equal.

FIG. 5 shows an alternative equalization method based on BER for a WDMsystem with J (J>2) optical channels propagating in the same direction,called herein the extrapolation method, as shown for example in FIG. 7for four channels. The required output power for a channel (j), wherejε[1, J] could be determined using the relation:

P(j)_(T) =P _(Max)−η(A(j)−A _(Min))  (5)

where the distance to failure A(j) of channel (j) is calculated as inEq(1) and Eq(2), namely A(j)=P_(Max)−P(j)_(Fail), where P_(Max) is themaximum output power of transmitter T_(j), P(j)_(Fail) is the transmitoutput power of channel (j) at which the BER reaches the predefinedBER_(Fail) value. A_(Min) in Eq. (5) is the minimum of A(j), and η is acoefficient depending on the number of channels. For example, theoptimal value for this coefficient, for a 3 or 4λ system is η=0.8.

To implement this extrapolation method, the following steps areperformed, as illustrated in FIG. 5.

As in the case shown in FIG. 4, the transmission link isinstalled/commissioned in step 100, T_(j)−R_(j) pairs are identified instep 105. and the BER for each channel in the failure point isidentified, which depends on the receiver. For example, different valuesare provided for the receiver of a OC-192 channel than for a receiverfor a OC-48 channel. This is shown in step 110.

Thereafter, all transmitters are set to a maximum output power in step115.

Next, the distance to failure in terms of attenuation for all (J)channels is determined in steps 180-205. When the system is providedwith an external monitoring network, such as is service channel 22 andmonitoring unit 24, the power monitors screens of receivers R_(j) may bebrought-up at terminal 11 site for reading the power values for allchannels. The output power is reduced for the first transmitter untilthis channel fails, i.e. a BER_(Fail) of 10⁻⁸−10⁻⁹ is reached at theoutput of R₁. It is to be noted that the value of BER_(Fail) has beenselected in the above range as an example only, other targets may beused in a similar way. This is illustrated by step 185 for channel λ(j).After the P(j)_(Fail) is recorded, the output power of T_(j) is reset toP_(Max), as shown in step 190.

Monitoring unit 24 determines the distance to failure A(j) for channel(1) according to Eq(1) in step 195. Steps 185 to 195 are repeated forall (J) channels, as shown by 200 and 205.

When all distances to failure are available, the minimum A(j) isdetermined in step 210 and denoted with A_(Min). Equalization ofchannels is next performed by adjusting the output power of alltransmitters according to Eq. (5), as shown by step 220. Step 220 isrepeated for all J channels, as shown at 225 and 230. In steps 240decision is made if the channels should be further equalized (finetuning) or not. Thus, for fine tuning, steps 185 to 240 are repeatedwith the powers found in step 220, as indicated in box 235.

FIG. 6 illustrates an example of a network provided with an externalmonitoring network. In the example of FIG. 6, the external monitoringnetwork comprises a service channel 22 and a monitoring unit 24, but theexternal monitoring may be effected in any other way. It is also to beunderstood that service channel 22 can be carried through the same fiberwith the user traffic, or may be a separate communication network.

The equalization for the link between transmitter terminal 11 andreceiver terminal 17 is performed automatically, in that the BERmeasured in the failing point of the channels and the powers of thetransmitters are processed by monitoring unit 24 which calculates theattenuations A(1) or A(2) and adjusts the operating point oftransmitters T₁ and T₂ accordingly. The service channel 22 conveys themeasured BERs to unit 24 processor that calculates the power margins andperforms the required adjustments of the power for each transmitter.

In an optically amplified system as shown in FIG. 6, the power margin ina point of interest may be measured using attenuators. As well,attenuators may be used for the case when the WDM system is equippedwith fixed power transmitters.

For measuring the received power margin of the entire transmission linkbetween terminals 11 and 17, a variable-optical-attenuator (VOA) 21 isconnected just before demultiplexer 15. The attenuation of the VOA 21for channel λ(1) is increased until the BER(1) reaches the failurecondition denoted with BER_(Fail). The transmitter powers are adjusteduntil all channels fail at the same VOA attenuation, in which case allchannels will have the same power margin.

Similarly, amplifier power margin can be measured by monitoring the BERat the output of a channel as the power of one or more amplifiers isreduced. The span margin can be measured by inserting a VOA into one ormore spans.

The transmitter power margin can be measured by setting all transmitoutput powers at maximum, then reducing the powers, one channel at atime until that channel fails. The measured distance to failure, interms of attenuation or power level, is then used to select the biasinglevels for the transmitters.

The various margins are very useful in that they give the network ownera good measure of how much more equipment/fiber can be added to thelink, or how the link may be reconfigured or upgraded. The presence ofthe external monitoring network simplifies the adjustment, in that allpower values measured for the system and the corresponding BERs may becollected in the point of interest, and also, the adjustment of anynetwork element may be effected remotely by the monitoring unit 24 basedon the data collected from the elements of the link.

The transmitter power adjustment may be done manually or automatically,using a model that predicts how the BER varies with the input power,while incorporating other relevant system constraints. With a controlfeedback loop through monitoring unit 24, the transmitter power marginsof the co-propagating channels can be equalized automatically, as themeasurements can be effected by the power monitor.

In case of more channels, the BER of all the channels can be equalizedby adjusting the transmitted powers of the various channels taking intoaccount the transmitter dynamic ranges, the system input power dynamicrange, and the measured margins to failure. The margins for the newvalues of input powers can be re-measured and the input powers can beiteratively adjusted to obtain a more accurate bias point. For mostcases, adequate equalization can be obtained by going through only oneor two iterations.

FIG. 7 depicts a schematic diagram of a typical WDM transmission linkwith multiple terminals. An Add/Drop multiplexer (ADM) 65 drops signalS₁′ travelling on channel λ(1) from the multi-channel signal S. ReceiverR₁ is located at the site of ADM 65 in this example. A fifth signal, S₅,which has the same band as signal S₁, is added on channel λ₁ tomulti-channel signal S, and transmitted from ADM 65 to terminal 17 whereit is detected by a receiver R₅.

Equalization of a network as shown in FIG. 7 or a network with anynumber of channels to be dropped/added at the ADM site may be performedusing the principle illustrated in FIG. 5 and disclosed in theaccompanying text.

A first way of equalizing performance of this network is shown in FIG.8A. After the network is installed/commissioned, as shown in step 100,all (K) channels to be dropped at the ADM 65 site, here designated with(k), are connected through the ADM coupler so as to by-pass the ADM.More precisely, the drop port and the add port of the coupler areconnected using a short patch-cord for each of the channels to bedropped, so that the channels travel all the way from terminal 11 toterminal 17. In the example of FIG. 7, channel S1 dropped at ADM 65 israther connected to by-pass the ADM to arrive at terminal 17. This isshown in step 300.

Steps 302 and 305 indicate that the procedure marked in FIG. 5 with A,performed for equalizing the performance of J end-to-end channels (fourin this example) are now carried for signals S₁-S₄ transmitted byterminal 11.

Next, ADM 65 is reconnected so that the K drop channels end at thatsite, as shown in step 310. In the example of FIG. 7, channel S₁ isreconnected to the drop port of ADM 65.

The power of each drop channels (k) is measured in step 320, and thechannel is added back at the ADM site, with the transmit power equal tothe received power, step 325. Steps 320 and 325 are repeated for alldropped channels, as shown at 315-335. In the present case, the power ofadded signal S₅ transmitted over channel λ₁ is adjusted to be equal tothe power of signal S₁′ measured at the ADM site.

FIG. 8B shows another variant of how the system of FIG. 7 can beequalized. In this variant, optical paths (a), (b) and (c) shown on thisfigure are considered namely: path (a), includes M channels directlyconnecting terminal 11 with terminal 17, passing through ADM 65; path(b) includes K channels connecting terminal 11 with ADM 65, and droppedat the ADM site; and path (c) includes K channels added at ADM 65,between ADM site and terminal 17. In this example, the number ofchannels dropped and added as equal.

This method employs again the procedure in FIG. 5 for optical path (a)and (b) with the channels dropped/added at the ADM site. FIG. 8B showsstep 100, whereby the link between terminals 11 and 17 isinstalled/commissioned, followed by equalization of all M direct(end-to-end) channels on link (a), as shown in steps 400 and 405. Insteps 410 and 415 channels K dropped at ADM 65 are equalized using thesame procedure as in FIG. 5. Then, the power of each of the K received(dropped) channels is measured at the ADM site, as shown in step 425.

Finally, at the ADM site, the transmit power of each added channelP(k)_(a) is set equal to the power of the corresponding dropped channelP(k)_(d), shown in step 425.

Another method of equalization of performance for the link of FIG. 7 isdisclosed next in connection with FIG. 8C. The procedure in FIG. 5 isapplied to optical paths (a), for all M direct channels betweenterminals 11 and 17, in steps 500-505, then to path (b) including all Kdropped channels, in steps 510-515 and then to path (c) including all Kadded channels, in steps 520-525, by considering the channels added backto the link through the ADM coupler as extra channels when using Eq(5).

To equalize a WDM network with multiple transmitter and receiverterminals, external monitoring network iterates the steps above untilthe powers of all channels converge to within predetermined guidelines.Since one or more channels may be operating error-free beforeequalization, the performance of these channels is degraded duringequalization to measure the received power margin, transmitted powermargin, amplifier power margin, or span margin.

The method may be used for network with channels of different bit rates.Monitoring unit 24 equalizes the BER at the receivers by adjusting thetransmitter powers, either by reducing the power of the strongesttransmitter, or by increasing the power of the weaker transmitters.

It is recommendable to carry out the equalization procedure when a newmulti-channel system is initially set up or upgraded to more channels.The method ensures that channels within each transmission band arematched in terms of BER performance. This procedure is particularlysuited for transmitters having adjustable output power, but can be alsoused for fixed power transmitters, by using VOAs to adjust thetransmitter powers.

For a multi-channel system, it is possible that one or more channelsoperate before equalization at a BER higher than the specifiedBER_(Fail). This happens because the ripple and gain tilt introduced byoptical amplifiers in the path of the optical signal vary with thewavelength. There are two ways to approach this problem. A firstsolution is to redefine the failure point and continue with the methoddescribed above. As indicated above, failure point is a user pre-definedpoint, so that a higher BER value may be selected for defining thefailing point, to operate all channels above it. Another solution is toattenuate the transmitter output power of all other channels, which areinitially running at a BER lower than the failure point. In this way,the performance of the failed channel(s) is brought back in the vicinityof the failure point. The output power of these transmitters is thenused as the initial setting.

Another way to define failure of a channel is to use a loss-of signal(LOS) or signal-degradation (SD) alarms at the receiver. Thisalternative method has the advantage that the LOS or SD alarm are raisedautomatically and immediately, so it gives a faster measure of thechannel failure point. On the other hand, this second alternative hassome drawbacks, such as: (1) the equalization would be performed withthe channels functioning away from the operating point so that theresults are less accurate, (2) it requires a larger transmitter powerdynamic range to reach the failure point, and (3) the LOS or SD alarmare not as accurate a measure of system performance as is the BER.

While the invention has been described with reference to particularexample embodiments, further modifications and improvements which willoccur to those skilled in the art, may be made within the purview of theappended claims, without departing from the scope of the invention inits broader aspect.

We claim:
 1. A method for equalizing a plurality of (J) signals S(j),travelling on a WDM link between a first terminal and a second terminalcomprising the steps of: for each channel λ(j) of said WDM link,measuring a distance to failure A(j) for a parameter P(j) of a signalS(j) travelling on said channel λ(j); and adjusting said parameter P(j)at said first terminal for obtaining equal distances to failure for allJ channels.
 2. A method as claimed in claim 1, wherein said step ofmeasuring comprises: inserting an attenuator in a point of interestbetween said first and said second terminal; and increasing theattenuation of said signal S(j) with said attenuator, until an errorcount indicator at said second terminal is higher than a provisionedthreshold.
 3. A method as claimed in claim 2, wherein said point ofinterest is the input of said second terminal and said parameter is thepower of said S(j) so as to equalize the power of all said signals S(j)at said second terminal.
 4. A method as claimed in claim 1, wherein saidpoint of interest is the output of an optical amplifier connected insaid WDM link, and said parameter is the gain introduced by saidamplifier on said signals S(j) at said second terminal.
 5. A method asclaimed in claim 1, wherein said point of interest is a span of said WDMlink and said parameter is the span loss for said signals S(j) measuredat said second terminal.